1. Field of the Invention
The present invention relates to a plant control system.
2. Description of the Prior Art
It is desirable from the standpoint of environmental protection that systems for purifying an exhaust gas emitted from internal combustion engines on automobiles, for example, with a catalytic converter such as a three-way catalytic converter and discharging a purified exhaust gas control the air-fuel ratio of an exhaust gas emitted from an internal combustion engine and introduced into the catalytic converter at an appropriate air-fuel ratio which allows the catalytic converter to have a better ability to purify an exhaust gas. The air-fuel ratio of the exhaust gas is more accurately the ratio of air to fuel in an air-fuel mixture which generates the exhaust gas when combusted.
One conventional air-fuel ratio control system combined with an internal combustion engine has been disclosed in Japanese laid-open patent publication No. 5-321721 which corresponds to U.S. Pat. No. 5,426,935.
The disclosed air-fuel ratio control system has an exhaust gas sensor (O.sub.2 sensor) disposed downstream of a catalytic converter for detecting the concentration of oxygen contained in an exhaust gas which has passed through the catalytic converter. The air-fuel ratio control system determines a target air-fuel ratio for the exhaust gas upstream of the catalytic converter according to a PID (proportional plus integral plus derivative) control process such that the oxygen concentration detected by the exhaust gas sensor will be of a predetermined target value. The air-fuel ratio control system then controls the internal combustion engine according to the target air-fuel ratio thereby to place the air-fuel ratio of the exhaust gas introduced into the catalytic converter (the air-fuel ratio of the air-fuel mixture to be combusted by the internal combustion engine) within a given range or window which enables the catalytic converter to have a good purifying ability.
In the above conventional air-fuel ratio control system, the exhaust system, including the catalytic converter, which ranges from a position upstream of the catalytic converter to a position downstream of the catalytic converter may be considered to be a plant for generating and emitting an exhaust gas having an oxygen concentration detected by the exhaust gas sensor, from an exhaust gas having a certain air-fuel ratio. The internal combustion engine may be considered to be an actuator for generating and emitting an exhaust gas having an air-fuel ratio to be supplied to the plant. Thus, the air-fuel ratio control system may be expressed as a system for determining a target input for the plant (more generally, a manipulated variable which defines an input to the plant) such that an output from the exhaust gas sensor (an oxygen concentration of the exhaust gas) as an output from the plant will be equalized to a given target value, and controlling an output of the internal combustion engine (=an input to the plant) as the actuator according to the target input.
As a result of various studies made by the inventors, it has been found that in order to keep the catalytic converter maximally effective to purify the exhaust gas regardless of aging thereof, it is necessary to adjust the concentration of a certain component, e.g., the concentration of oxygen, of the exhaust gas downstream of the catalytic converter to a predetermined target value with high accuracy. In the above conventional air-fuel ratio control system based on the PID control process, it is difficult to adjust the oxygen concentration of the exhaust gas having passed through the catalytic converter highly accurately to a given target value because of disturbances and a dead time present in the exhaust system of the internal combustion engine.
The inventors have devised a control system for compensating for, or eliminating, the effect of such a dead time (see, for example, U.S. patent application Ser. No. 08/835,192 (Japanese laid-open patent publication No. 9-273438)).
The above control system has a sensor disposed upstream of the catalytic converter for detecting the air-fuel ratio of an exhaust gas introduced into the catalytic converter, in addition to the exhaust gas sensor disposed downstream of the catalytic converter. In the control system, the exhaust system including the catalytic converter and ranging from the upstream sensor to the downstream sensor is modeled as a continuous system (specifically, a continuous time system). Based on the model, an output from the downstream sensor is estimated from outputs of both the sensors after a dead time of the exhaust system, and the air-fuel ratio of the exhaust gas introduced into the catalytic converter, i.e., the air-fuel ratio of an air-fuel mixture combusted by the internal combustion engine, is controlled in order to adjust the estimated value to the target value.
It is possible according to the above control process to eliminate the effect of the dead time present in the exhaust system including the catalytic converter for thereby stably controlling the air-fuel ratio of the exhaust gas emitted from the internal combustion engine.
According to the above control system, since the exhaust system from the sensor upstream of the catalytic converter to the sensor downstream of the catalytic converter is modeled as a continuous system in order to estimate an output from the downstream sensor after the dead time, an algorithm for estimating an output from the downstream sensor is constructed on the continuous system model. However, because a computer for executing the algorithm can only perform discrete-time processing, it is tedious and time-consuming to estimate an output from the downstream sensor.
Furthermore, inasmuch as the exhaust system including the catalytic converter is modeled as a continuous system, it is difficult to set parameters, including gain coefficients, of the continuous system model in a manner to match various operating conditions of the exhaust system, and hence to model the exhaust system with accuracy. If an error between the model and the actual exhaust system, i.e., a modeling error, is too large, then the estimated output from the downstream sensor may be of an inappropriate value which does not match the actual exhaust system. As a result, it is difficult to carry out the control process stably for adjusting the output from the sensor downstream of the catalytic converter to the given target value.
The above drawbacks are not limited to the above air-fuel ratio control system, but may also be addressed to any arbitrary system in which in order to control an output from a plant having a relatively long dead time at a predetermined target value, the plant is modeled as a continuous system and an output from the plant after the dead time is estimated on the basis of the plant.